Kin Man Ho

Design and synthesis of novel magnetic core-shell polymeric particles.

A novel synthetic strategy was developed for the preparation of magnetic core-shell (MCS) particles consisting of hydrophobic poly(methyl methacrylate) cores with hydrophilic chitosan shells and gamma-Fe2O3 nanoparticles inside the cores via copolymerization of methyl methacrylate from chitosan in the presence of vinyl-coated gamma-Fe2O3 nanoparticles. The magnetic core-shell particles were characterized with transmission electron microscopy, field-emission scanning electron microscopy, particle size and zeta-potential measurements, vibrating sample magnetometry, and atomic force microscopy, respectively. The MCS particles were less than 200 nm in diameter with a narrow size distribution (polydispersity = 1.09) and had a good colloidal stability (critical coagulation concentration = 1.2 M NaCl at pH 6.0). Magnetization study of the particles indicated that they exhibited superparamagnetism at room temperature and had a saturation magnetization of 2.7 A m2/kg. The MCS particles were able to form a continuous film on a glass substrate, where magnetic nanoparticles could evenly disperse throughout the film. Thus, these new materials should be extremely useful in various applications.

Facile route to enzyme immobilization: core-shell nanoenzyme particles consisting of well-defined poly(methyl methacrylate) cores and cellulase shells.

A one-step method for preparing cellulase-immobilized nanoparticles that consist of well-defined poly(methyl methacrylate) (PMMA) cores and cellulase shells has been developed. The core-shell nanoparticles are synthesized from a direct graft copolymerization of methyl methacrylate (MMA) from cellulase in an aqueous medium. Particle formation strongly depends on the surface nature of the cellulase (e.g., pH of reaction media) and MMA to cellulase weight ratio. Under optimized conditions, high MMA conversions (>90%) were achieved, and the PMMA-cellulase nanoparticles produced were very stable with narrow size distributions ( Dv/Dn < 1.20). Particle sizes in the range between 80 and 124 nm (volume average diameter) could be tailored by a variation of cellulase concentration. Transmission electron microscopy micrographs revealed that the nanoparticle had a well-defined PMMA core which was evenly coated with cellulase shell. Study of cellulase activity of the PMMA-cellulase nanoparticles indicated that even though activity of immobilized cellulase on the nanoparticles was 41% less than that of the native cellulase after the polymerization, the immobilized cellulase showed improved properties such as broader working pH range and better thermal stability. Other important advantages of this approach include that the PMMA-cellulase nanoparticles could be produced in high concentrations (up to 18% w/w solids content) and the nanoparticles have thick and evenly distributed enzyme shells. Thus, this method may provide a new commercially viable route to the immobilization of thermally stable enzyme to form nanoenzyme particles.

Polyethyleneimine-Based Core-Shell Nanogels: A Promising siRNA Carrier for Argininosuccinate Synthetase mRNA Knockdown in HeLa Cells

RNA interference using small interfering RNA (siRNA) is a promising biological strategy for treatment of diverse diseases; however, application of siRNA is severely hindered by its poor stability and low cellular uptake efficiency. We have recently demonstrated that polyethyleneimine (PEI)-based amphiphilic core-shell particles have several distinguishing advantages over native PEI and its derivatives. This paper presents a novel type of PEI-based nanogels with a biodegradable gelatin core. The core-shell nanogels were synthesized via a two-stage reaction: (1) preparation of highly uniform gelatin nanoparticles through appropriate treatment of gelatin solution; and (2) conjugation of branched PEI to the preformed gelatin nanoparticles, followed by repeated cycles of desolvation and drying of the gelatin-PEI nanogels in ethanol/water mixture. The resulting nanogels have a well-defined nanostructure that contains a gelatin core and a PEI shell. They have an average diameter of 200 ± 40nm with high uniformity. The nanogel particles possess positive zeta-potential values of up to +40mV at neutral pH, indicating that they are highly positive and very stable in aqueous medium. The gelatin-PEI nanogels were able to completely condense siRNA at N/P ratios of as low as 5:1, and effectively protected siRNA against enzymatic degradation. Furthermore, the nanogels were four times less toxic than native PEI. Besides low toxicity, the nanogels were able to effectively deliver siRNA into HeLa cells. It was found that increasing the N/P ratio from 10 to 30 significantly increased the intracellular uptake efficiency of siRNA from 41 to 84%. Confocal laser scanning microscopic images confirmed that the nanogels were able to effectively deliver siRNA in the cytoplasm of HeLa cells. The delivered siRNA could inhibit 70% of human argininosuccinate synthetase 1 (ASS1) gene expression. This gene silencing percentage is much higher than that of the commercial Lipofectamine(TM) 2000. Our studies demonstrate that gelatin-PEI core-shell nanogels have promising potential to act as an effective siRNA carrier.

Hydrothermal Microemulsion Synthesis of Oxidatively Stable Cobalt Nanocrystals Encapsulated in Surfactant/Polymer Complex Shells

Air-stable magnetic cobalt nanocrystals have been conveniently prepared via a reverse micellar synthesis, followed by a hydrothermal treatment. The synthesis was carried out by first mixing an aqueous solution containing cobalt chloride and poly(sodium 4-styrenesulfonate) (PSS) with an organic mixture containing cetyltrimethylammonium bromide (CTAB) to form reverse micelles, followed by reducing cobalt ions with sodium borohydride. The resultant nanoparticles were then undergone a hydrothermal treatment at 165 degrees C for 8 h to generate well-dispersed CTAB/PSS-encapsulated cobalt nanocrystals with an average diameter of 3.5 +/- 0.5 nm. The nanoparticles were highly crystalline with a hexagonal close-packed crystal phase. The presence of CTAB/PSS complex coatings was identified by FT-IR and UV-vis spectroscopies as well as thermogravimetry analyses. The nanocrystals exhibited superparamagnetic property at room temperature with a saturation magnetization (M(s)) of 95 emu/g. The magnetization could be largely preserved after storage at room temperature for 4 months as the M(s) value only slightly decreased to 88 emu/g (measured at 300 K). Thus, the polymer encapsulation could not only improve thermal stability of the micelles for the growth and nucleation of Co atoms but also protect the resulting cobalt nanocrystals from oxidation through forming an oxygen impermeable sheath.

Formation of Nanostructured Materials Using Inexpensive Hollow Particles of Amphiphilic Graft Copolymers As Building Blocks: 1. Insight into the Mechanism of Nanotube Formation

The aim of this research is to elucidate the mechanism for the formation of nanotubes from hollow particles of amphiphilic graft copolymers, such as poly(ethyleneimine)-graft-poly(methyl methacrylate) (PEI-g-PMMA), under non-equilibrium conditions. The study was performed at either 15 or 17 °C with fluid shear in a mixture of dichloromethane (DCM)–water using the amphiphilic hollow particle as a building block. Effects of stirring rate and DCM to water ratio on the hollow particle assembly were systematically investigated. Surface properties and morphology of the hollow particles and the resulting assemblies in both DCM and water were characterized by X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy and atomic force microscopy. Results from these studies suggest four key features of this assembly process: (1) morphology of the amphiphilic hollow particle can be inverted in organic solvent and water. (2) The assembly process can only occur with appropriate fluid shear and DCM:water ratio. (3) The hollow particles can undergo deformation to ellipsoidal shapes with stirring rate at 350 rpm in an appropriate DCM:water (e.g. 3:7 v/v) mixture. (4) The elongated hollow particles then assemble to form linear aggregates via tip-to-tip connection, followed by coalescence and fusion to generate nanotubes with diameters less than 150 nm. The lengths of the nanotubes can be up to micron-scale, and they can be easily aligned via a simple dip-coating method. This simple and inexpensive assembly process using amphiphilic hollow particles as a building blocks is dramatically different from the well-known self-assembly of block copolymers into different nanostructures under equilibrium conditions.